Elastic fluid turbine system

ABSTRACT

An auxiliary turbine having a steam admission control valve is provided in a turbine system for generating a reverse torque with respect to the main turbine unit. The steam admission control valve is actuated to open if the turbine speed rises above a predetermined value or at a rate of change above a predetermined value, whereby an undesirable turbine overspeed is prevented.

United States Patent [151 3,692,419 Katagiri et al. [4 1 Sept. 19, 1972 [5 ELASTIC FLUID TURBINE SYSTEM [56] References Cited [72] Inventors: Shigenobu Katagiri, Kanagawa-ken; UNITED STATES PATENTS Mitsuhisa Yokota, Yokohama, both f Japan 3,427,464 2/1969 Watson ..4l5/l7 [73] Assignee: Tokyo Sbibaura Electric Co., Ltd., primary c Husar Ka asak Japan Attorney0blon, Fisher & Spivak 22 F1 d: 3, 1971 TH 21 A l N 3:640 [57] ABS CT 1 pp 0" An auxiliary turbine having a steam admission control valve is provided in a turbine system for generating a [30] Foreign Application Priority Data reverse torque with respect to the main turbine unit. The steam admission control valve is actuated to open June 4, 1970 Japan ..45/47608 if the turbine Speed rises above a predetermined value or at a rate of change above a predetermined value, lll "4154533); whereby an undesirable turbine overspeed is 58 Field of Search ..415/13, 17, 30 prevented 9 Claims, 4 Drawing figures PATENTEDSEP 19 m2 3.692.419

SHEET 1 0F 2 -1 GENERATOR FIG. 1

' GENERATOR FIG. 2

INVENTORS SHIGENOBU KATAGIRI MITSUHISA YOKOTA ATTORNEY ELASTIC FLUID TURBINE SYSTEM BACKGROUND OF THE INVENTION 1. Field of the Invention:

This invention relates generally to an elastic fluid turbine system and more particularly to a turbine system of the type adapted to be operated by a motive fluid having relatively low energy density.

2. Prior Art of the Invention:

In a turbine driven by a saturated steam of relatively low temperature and pressure, certain problems result from a drain or the moisture being entrained in the steam. In order to remove the moisture from the steam, moisture separators have been utilized, but even with these, it has been impossible to separate the moisture from the steam completely.

When the turbine speed rises above a predetermined level because of a sudden reduction of the turbine load, the steam flow to the turbine is stopped. For example, a main control valve, which controls the admission of steam to a high pressure turbine, is generally closed entirely at substantially 105 percent of the rated speed, and an intercept valve, which controls steam flow from the high pressure turbine to a low pressure tubine, is closed entirely at substantially 102.5 percent of the rated speed. After closure of these valves, the pressure of the fluid in the stage below the intercept valve, such as the low pressure turbine, becomes substantially equal to that of the condenser, such as, for example, 38

mm Hg, in just a few minutes. Then the surviving moisture or drainage again is vaporized under the changed condition of saturation pressure and temperature. The steam being revaporized in the upper stream of the low pressure turbine expands therein and flows through the various stages to the condenser.

Hence, there was the fear, after the full closure of the intercept valve, that such revaporized steam might have a large enough energy to cause the turbine to accelerate to an unfavorable overspeed condition, such as, for example, about 1 percent of the rated speed, at which speed an emergency trip switch will operate. Once the emergency trip operates, it is troublesome to start up the turbine system again.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide an elastic fluid turbine system capable of preventing an unfavorable overspeed.

Another object of this invention is to provide a braking means which operates quickly and reliably to prevent an unfavorable overspeed of the turbine system.

Still another object of the present invention is to provide a novel and improved control means for actuating the braking means at a predetermined overspeed.

A still further object of the invention is to provide a control means for actuating a braking means if the turbine speed accelerates at a higher rate than a preselected value.

Briefly stated, in a turbine system having high and low pressure turbines, the foregoing objects are attained by providing a reverse torque generating turbine for braking against a rotational output of the high and low turbines. There are also provided a braking control valve for controlling admission of the motive fluid to the reverse turbine and a control means for the braking control valve. If the turbine speed rises above a predetermined level or accelerates at a rate above a predetermined level under the control of the high and low pressure turbines, the braking control valve automatically actuates to open so that the reverse turbine generates a braking force to prevent an undesirable overspeed condition from arising.

BRIEF DESCRIPTION OF THE DRAWING Various other objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood from the following detailed description when considered in connection with the accompanyings drawings, wherein:

FIG. I shows a schematic diagram of one embodiment of an improved turbine system in accordance with the present invention;

FIG. 2 is a schematic diagram of another embodiment of a turbine system according to the present invention;

FIG. 3 is a schematic diagram of one embodiment of a control means for the system according to this invention; and

FIG. 4 illustrates a schematic diagram of another embodiment of a control means for the system according to the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several illustrated embodiments, and more particularly to FIG. 1 thereof, a steam generated by a boiler 1 flows through a control valve 2 to a high pressure turbine 3 and expands therein, and then flows to a moisture separator 4 of conventional design wherein a certain portion of the moisture entrained in the steam is removed. The stem then flows through an intercept valve 8 and a cross-over line 9 to a low pressure turbine stage 5 including low pressure tur bines 6 and 7, and then to a condenser 10. The high and low pressure turbines 3 and 6 and 7, respectively, have a common load such as an A.C. generator I I.

In this embodiment there is further provided a reverse torque generating turbine 12 coupled to a common shaft of the high and low pressure turbines 3, 6, and 7, hereinafter referred to as the main turbine unit, for braking the rotational output thereof. Maximum output of the reverse turbine 12 may be established at about 10 percent of the output of the main turbine unit. The motive fluid for the reverse turbine 12 is supplied from the boiler 1 under the control of a braking control valve 13 provided adjacent the motive fluid inlet of the reverse turbine 12.

In normal operation, the braking control valve 13 is fully closed, but should the turbine speed rise to an undesirable overspeed condition, the braking control valve 13 will be opened by a control means which hereinafter will be described in connection with FIGS. 3 and 4. Hence, the reverse turbine generates a torque in the reverse rotational direction relative to the torque being generated by the main turbine unit to restrict the speed increase thereof resulting essentially from the revaporized steam in the lower stage turbines.

Referring now to FIG. 2, which illustrates another embodiment of the system constructed according to the present invention, a reverse torque generating turbine 12 similar to that shown in FIG. 1 is provided, but in this case the motive fluid for driving the reverse turbine 12 is supplied from the main steam line being taken from a point downstream from the intercept valve 8, such as the cross-over passage 9, through a braking control valve 13. In this embodiment when the braking control valve 13 is actuated to an open position by the control means, as will hereinafter be explained, the revaporized steam in the low pressure turbine stage flows to the reverse turbine 12 as well as to the low pressure turbines 6 and 7. The torque in the reverse rotational direction generated by the reverse turbine 12 accordingly is operative to restrict the speed rise of the system.

ln these embodiments of the system, it is preferable to position the braking control valve 13 as near the reverse turbine 12 as possible in order to decrease the time lag and to generate a braking force in quick response to the actuation of valve 13, wherein the same is opened.

Referring now to FIG. 3, there is illustrated an embodiment of a servomechanism for controlling the braking control valve 13. Generally stated, there is provided a governor which is independent of the main governor for controlling the main control valve 2 and the intercept valve 8 and is designed such that after the control and intercept valves have been entirely closed, the valve 13 begins to open at a turbine rotational speed of about 106 percent, for example, of the rated speed and reaches its full opening at about 108.5 percent, for example, of the rated speed. Accordingly, after the control and intercept valves have been closed at a turbine speed above approximately 105 percent of the rated speed, if the turbine speed continues to rise substantially, the braking control valve 13 begins to open so that the steam flows into the reverse turbine 12 to generate the necessary braking force.

The reference numeral 14 indicates a worm coupled to the turbine shaft and operatively engaged with worm wheel 15 mounted on a shaft of a conventional main speed governor 16 which operates the control and intercept valves in a conventional manner. The governor 16 moves its operating rod 17 up and down, respectively, in response to lower and higher speeds of rotation of the wheel 15. The operating rod 17 is coupled to a piston rod having thereon two lands of a pilot valve 18. This valve 18 has a sleeve or a cylinder 19 including a top port for drain, a bottom inlet port for receiving a hydraulic fluid, and an intermediate port 20. The intermediate port 20 is connected to a lower chamber of a servomotor 21 having a stationary cylinder in which a piston 22 is slidably disposed, being forced toward one end of the cylinder, or downward, as illustrated, by a compressed spring 23. Upper and lower rods 24 and 25 are secured to the piston 22, the lower rod 25 being operatively connected to the control and intercept valves 2 and 8, shown in FIG. 1, through a suitable servomechanism, not shown, whereby the servomotor 21 functions as a speed relay.

A speed/load changer, or load setting device, 26 is provided for controlling the position of the right end of a lever 27, which is linked at its left end to the upper rod 24 of the servomotor 21 and at an intermediate point to the cylinder 19 of the pilot valve 18 through a connecting rod.

Meshing with the worm 14 is a worm wheel 28 to operate a secondary speed governor 29 having an operating rod 30 which moves up and down, respectively, in response to lower and higher speeds. This operating rod 30 is connected to the piston rod of a pilot valve 31. The pilot valve 31 has an upper inlet port for receiving a hydraulic fluid and a lower port 32 which is connected to the lower chamber of a servomotor 33 having a stationary cylinder and a piston 34 with a rod 35. The piston 34 is biased downwardly by a biasing spring 36. The movement of the rod 35 is fed back to the sleeve of the pilot valve 31 through a restoring linkage including a lever 37 with a fulcrum at an intermediate point thereof, a link 38 and a lever 39. The left end of the lever 39 is positioned by a setting device 40 to determine the speed at which the braking control valve begins to open.

Movement of the rod 35 is also transmitted to a lever 41 mounted fixedly on one end of a torque imparting shaft 42 which is rotatably supported by a suitable means. The braking control valve 13 and a servo device for actuating the same are disposed substantially apart from the servomotor 33. ln order to transmit the movement of the servomotor rod 35 to the braking control valve servo device, the torque imparting shaft 42 is employed. Therefore, other transmission means, such as a hydraulic system, may be used. Mounted fixedly on the other end of the shaft 42 is a lever 43 which links operatively to a piston rod 44 of a pilot valve 45 through a link 46 and a lever 47.

The pilot valve 45 has a stationary sleeve having an upper port for drain, a lower inlet port 48 for receiving a hydraulic fluid and an intermediate port 49 which is connected to the bottom chamber of a servomotor S0 for operating the braking control valve 13. The servomotor 50 has a stationary cylinder and a piston 51 which is biased downward by a compressed spring 52. A rod 53 on the piston 51 is connected to the stem of the valve 13. There is also provided a restoring linkage including a lever 54 having a fulcrum at an intermediate point thereof, a link 55, and the lever 47 in order to feed back the movement of the servomotor rod 53 to the piston rod 44 of the pilot valve 45.

in order to operate the brake valve 13 rapidly and reliably, it is necessary to provide an adequate feed of the hydraulic fluid. To this end, there is provided an accumulator 56 supplied with the hydraulic fluid through a line 57 having an orifice 58 therein and also supplying the inlet port 48 of the pilot valve 45 with the fluid through a line 59.

In a normal operation, the rod 25 of the speed relay 21 moves up and down in response to speeds which are respectively lower and higher than the rated speed to actuate the control and intercept valves 2 and 8, during which time the brake valve 13 is in a completely closed condition.

When the turbine speed increases to about 102.5 percent of its rated speed, the intercept valve 8 becomes fully closed and when the speed increases further to about percent of the rated speed the control valve 2 also is entirely closed. At the same time the operating rod 30 of the governor 29 moves downward. When the turbine speed continues to increase further over about I06 percent of the rated speed, the piston stem of the pilot valve 31 is moved downward sufficiently far to communicate the bottom of the servomotor 33 with the hydraulic source through the area between the lands of the pilot valve 31 and the port 32. Accordingly, the piston 34 is moved upward to an equilibrium point thus causing the piston rod of the pilot valve 45 also to shift upwardly. The lower chamber of the servomotor 50 then receives hydraulic fluid from the accumulator 56 through line 59, inlet port 48 of pilot valve 45 and port 49, whereby the braking control valve 13 begins to open as the piston 51 and rod 53 thereof are moved upwardly. In this manner the braking control valve 13 may be actuated to open and close in accordance with the turbine speed under the control of the governor 29. The brake control valve 13 preferably is adapted to attain a fully open position if the turbine speed reaches 108.5 percent of its rated speed.

In the case where the turbine system shown in FIG. I is controlled by this embodiment, as the valve 13 opens at a speed above 106 percent of the rated speed of the system, with the control and intercept valves being entirely closed, the steam flows from boiler 1 to the reverse turbine 12 to generate a braking force so that the speed increase can be kept below the undesirable overspeed. This result is possible because a sufficient quantity of steam is present in the boiler to overcome the torque in the normal direction caused by the revaporized steam, even though the production of steam in the boiler has been stopped as the control and intercept valves were closed.

Thus the turbine speed decreases toward the rated speed whereby the braking control valve 13 is returned gradually toward its closed state and finally to its fully closed state under the force of the spring 52.

In the case where the turbine system shown in FIG. 2 is controlled by this embodiment, after the valve 13 has been opened, the control and intercept valves already having been closed, the revaporized steam in the low pressure stage 5 flows to the reverse turbine 12 as well as to the low pressure stage. Accordingly, the reverse turbine 12 generates a torque in the reverse direction to restrict the speed increase whereby the system is prevented from reaching an undesirable overspeed condition.

It will be understood that the speed at which the braking control valve 13 begins to open may be adjusted by the setting device 40 for changing the position of the left end of the lever 39. The speed at which the valve 13 reaches its fully open position is determined by the design of the restoring linkage. It is preferable of course that these setting speeds be adjusted to effectively prevent an undesirably overspeed from being reached, according to the characteristics of the particular system involved.

According to this embodiment, as there is provided a speed governor which is independent from the main speed governor, the braking control valve can be controlled in a desirable manner so as to prevent the undesirable overspeed from occurring. Further, the provision of the accumulator for supplying enough hydraulic fluid to the servo device for actuating the brake control valve permits a rapid and effective generation of the braking force to be applied in response to the control means.

FIG. 4 shows another embodiment of the control means formed according to this invention. In this embodiment, the braking control valve 13 is operated to open when the intercept valve 8 closes with a rate of change over a prescribed value. In order to detect the rate of change, a differential element including a dashpot and a spring is provided.

It is noted that although FIG. 4 illustrates the control system being adapted for use with the turbine system shown in FIG. 2 in that the inlet of the brake valve 13 is connected in the steam line below the intercept valve 8, this embodiment also is applicable to the system shown in FIG. 1.

In FIG. 4 the intercept valve 8 is operated by a servomotor 60 through a lever 61 being fulcrumed at an intermediate point thereof. The servomotor 60 has a stationary cylinder and a piston 62 therein which is biased downwardly by a biasing spring 63. A rod 64 of the piston 62 is operatively connected to the lever 61 and has two arms 65 and 66. The servomotor 60 is actuated by a pilot valve 67 having a stationary sleeve. This sleeve has an upper port for a drain line, a lower inlet port for receiving a hydraulic fluid and an intermediate port 68 connected to the top chamber of the servomotor 60. A rod 69 of a spool piston of the pilot valve 67 is linked to the intermediate portion of a lever 70, the left end of which is operatively connected to a speed relay, such as the servomotor 21 shown in FIG. 3, through a suitable servomechanism. The movement of the piston rod 64 of the servomotor 60 is fed back to the valve stem 69 through a restoring linkage consisting of the arm 65, a link 71 and the lever 70. Accordingly, the intercept valve 8 is actuated to open and close in accordance with the speed relay, not shown.

Movement of the piston rod 64 of the servomotor 60, which in turn, through the lever 61, means the movement simultaneously of the intercept valve 8, is transmitted to a differential element 72 through the arm 66, a link 73, a lever 74 fulcrumed at the intermediate portion thereof and a link 75.

Next, the differential element will be explained. There is provided a stationary fluid dashpot 76 having a piston 77 therein. An upper chamber of the dashpot 76 communicates with a lower chamber thereof through a by-pass line 78 having a needle valve 79 therein. A rod of the piston 77 links to a lever 80 at its intermediate portion 80b. The left end 80a of the lever 80 is operatively engaged with the link 75. There are also provided an upper and a lower spring, 81 and 82, respectively, for resiliently supporting the right end 80c of the lever 80 in a neutral position.

A movement of the right end 80c of the lever 80, which is the output of the differential element 72, is transmitted through a link 83 and a lever 84 to a pilot valve 85 having a stationary sleeve with an upper port for a drain, a lower inlet port for receiving a hydraulic fluid and an intermediate port 86. The intermediate port 86 communicates with a lower chamber of a servomotor 87 for actuating the braking control valve 13. A piston 88 with a rod 89 connected to the valve stem of the valve 13 is forced downward by a biasing spring 90. Movement of the stem 89 is fed back to the piston rod of the pilot valve 85 through an arm 91 of the stern 89, a link 92, a lever 93 with a fulcrum at an intermediate point thereof, a link 94 and the lever 84.

In operation, the intercept valve 8 is operated to open and close in accordance with a signal given to the left end of the lever 70. The operating speed of the intercept valve 8 depends on the rate of change of the turbine speed and the movement of the intercept valve 8 is transmitted to the lever 80 of the differential element 72 through the lever 61, the rod 64, arm 66, link 73, lever 74 and link 75. As the change in turbine speed under normal conditions is a relatively small quantity and is relatively slow, the moving speed of the left end 80a of the lever 80 is also slow. in such normal operation, the lever 80 moves about the right end 80c, which functions as a fulcrum, because the right end is substantially supported by the springs 81 and 82, and the piston 77 of the dashpot can follow the slow movement of the lever portion 80b. Accordingly, the pilot valve 85 keeps substantially on-port, as shown, or toward a slightly closing state, under normal operation. The braking control valve 13 thereby assumes a fully closed position.

In the event a relatively quick speed increase occurs upon the occurrence of some trouble, such as, for example, a loss of the load, the intercept valve 8 is actuated to close rapidly, The left end 80a of the lever 80 is thereby caused to move downward with a relatively rapid speed. As the piston 77 of the dashpot 76 cannot move quickly, the lever rotates about the point 80b being pivotally supported by the stem of the piston 77 against the resilient force of the springs 81 and 12. That is, the right end 80c moves upward, so that the spool of the pilot valve 85 is raised and the hydraulic fluid flows into the bottom of the servomotor 87 to open the braking control valve 13. Hence, the steam flows to the reverse turbine 12 to generate a braking force, whereby the turbine system is prevented from attaining an undesirable overspeed.

As the piston rod of the dashpot 76 is subjected to a force in the downward direction, the fluid below the piston 77 flows into the upper chamber of the dashpot through the needle valve 79 so that the piston 77 moves downward gradually, instead of at a fast rate. With this downward movement of the portion 80b of lever 80, the right end 80c thereof also moves downward to cause the lower chamber of the servomotor 87 to communicate with a drain line in the pilot valve whereby the brake valve 13 closes slowly.

In this manner, a condition in which the braking control valve 13 begins to open and the period in which the same remains in an open state are determined by the adjustment of the needle valve 79 and the constants of the springs 81 and 82. Hence, according to the actual system which may be either that shown in FIG. 1 or in FIG. 2, the needle valve and the springs are adjusted so as to prevent the undesirable overspeed condition of the turbine system.

In this embodiment, the differential element is provided for detecting a closing operation of the intercept valve at a relatively high rate of speed, such as may occur under a condition where it is necessary to open the braking control valve. Hence, the brake valve may be actuated to an open position in rapid response to such a detection so that the braking force can be immediately and effectively used.

According to this invention, there are provided a reverse torque generating turbine stage for obtaining a braking force and a control means for controlling the same, such that the turbine system is prevented from reaching an undesirable overspeed condition at which the emergency governor will operate. The system according to the present invention is especially useful in a plant where it is necessary to employ a wettable steam, namely, for example, an atomic power plant of the BWR type.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is to be understood, therefore, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

i. In an elastic fluid turbine system having a high pressure turbine stage, a low pressure turbine stage, a control valve for controlling the admission of a motive fluid to said high pressure turbine stage, an intercept valve for controlling the admission of motive fluid to said low pressure turbine stage, and a speed governor for said control and intercept valves, the improvement comprising:

a reverse torque generating turbine for braking the rotational output of said high and low pressure turbine stages;

a braking control valve for controlling the admission of motive fluid to said reverse torque generating turbine; and

a control means for controlling said braking control valve to prevent an undesirable rotational overspeed of said high and low pressure turbine stages.

2. An elastic fluid turbine system according to claim 1, wherein said braking control valve has an inlet connected to an inlet of said controi valve.

3. An elastic fluid turbine system according to claim 1, wherein said braking control valve has an inlet connected to an outlet of said intercept valve.

4. An elastic fluid turbine system according to claim 2, wherein said control means comprises an auxiliary speed governor for causing said braking valve to open upon the turbine speed increasing beyond a predetermined value.

5. An elastic fluid turbine system according to claim 3, wherein said control means comprises an auxiliary speed governor for causing said braking valve to open upon the turbine speed increasing beyond a predetermined value.

6. An elastic fluid turbine system according to claim 2, wherein said control means comprises a differential element for detecting a closing of said intercept valve at a relatively high rate of speed.

7. An elastic fluid turbine system according to claim 3, wherein said control means comprises a differential element for detecting a closing of said intercept valve at a relatively high rate of speed.

8. An elastic fluid turbine system according to claim 1, wherein said control means comprises:

means biasing said braking control valve to a closed position;

fluid pressure means for overcoming said biasing means to open said braking control valve;

a pilot valve for controlling said fluid pressure means;

means responsive to a turbine speed above a preselected level for operating said-pilot valve to cause said braking control valve to be opened; and

dashpot means for detecting an increase of said turbine speed at a high rate and being responsive thereto for operating said pilot valve to cause said braking control valve to be opened, but permitting said braking control valve to be slowly closed by said biasing means if said preselected level of speed is not exceeded. 

1. In an elastic fluid turbine system having a high pressure turbine stage, a low pressure turbine stage, a control valve for controlling the admission of a motive fluid to said high pressure turbine stage, an intercept valve for controlling the admission of motive fluid to said low pressure turbine stage, and a speed governor for said control and intercept valves, the improvement comprising: a reverse torque generating turbine for braking the rotational output of said high and low pressure turbine stages; a braking control valve for controlling the admission of motive fluid to said reverse torque generating turbine; and a control means for controlling said braking control valve to prevent an undesirable rotational overspeed of said high and low pressure turbine stages.
 2. An elastic fluid turbine system according to claim 1, wherein said braking control valve has an inlet connected to an inlet of said control valve.
 3. An elastic fluid turbine system according to claim 1, wherein said braking control valve has an inlet connected to an outlet of said intercept valve.
 4. An elastic fluid turbine system according to claim 2, wherein said control means comprises an auxiliary speed governor for causing said braking valve to open upon the turbine speed increasing beyond a predetermined value.
 5. An elastic fluid turbine system according to claim 3, wherein said control means comprises an auxiliary speed governor for causing said braking valve to open upon the turbine speed increasing beyond a predetermined value.
 6. An elastic fluid turbine system according to claim 2, wherein said control means comprises a differential element for detecting a closing of said intercept valve at a relatively high rate of speed.
 7. An elastic fluid turbine system according to claim 3, wherein said control means comprises a differential element for detecting a closing of said intercept valve at a relatively high rate of speed.
 8. An elastic fluid turbine system according to claim 1, wherein said control means comprises: means biasing said braking control valve to a closed position; fluid pressure means for overcoming said biasing means to open said braking control valve; a pilot valve for controlling the admission and discharge of fluid for respectively opening and closing said braking control valve; and means responsive to a predetermined turbine speed for operating said pilot valve to admit fluid pressure to said biasing means for overcoming the biasing force thereof and to open said braking control valve.
 9. An elastic fluid turbine system according to claim 1, wherein said control means comprises: biasing means for normally urging said braking control valve to a closed position; fluid pressure means for opposing said biasing means to open said braking control valve; a pilot valve for controlling said fluid pressure means; means responsive to a turbine speed above a preselected level for operating said pilot valve to cause said braking control valve to be opened; and dashpot means for detecting an increase of said turbine speed at a high rate and being responsive thereto for operating said pilot valve to cause said braking control valve to be opened, but permitting said braking control valve to be slowly closed by said biasing means if said preselected level of speed is not exceeded. 